1. Field of the Invention
The invention relates to adhesive compositions and, more specifically, to extrudable polyolefin-based adhesive compositions containing both acid (or acid derivative) functionality and silane functionality.
2. Description of the Prior Art
Adhesive blends, commonly referred to as tie layer adhesives, designed to improve adhesion between polyolefins and dissimilar substrates, such as polyamides and ethylene-vinyl alcohol (EVOH) copolymers, metals and the like, in multi-layer constructions are well known. These adhesive blends typically consist of a polyolefin base resin, which is the predominant component in the blend, and a modified polymer containing carboxylic acid or anhydride functionality, e.g., polymers grafted with maleic anhydride. Optionally, one or more other polymeric materials, including rubbers, are included in the adhesive blends. Representative adhesive blends of the above types are described in U.S. Pat. Nos. 4,087,587; 4,298,712; 4487885; 4774144; and 5367022.
While numerous tie-layer adhesive compositions have been developed, new formulations which extend the envelope of performance or meet the requirements of new laminate applications are always in demand. Tie-layer adhesives having improved adhesion, particularly at high temperatures, are especially desirable. High temperature conditions may occur as a result of the processing or manufacturing operations or the environment to which the finished article is exposed. Examples of the latter would be for multi-layer tubing for under-the-hood automotive applications where the lines can be exposed to high temperatures for extended periods and where adhesive compositions resistant to thermal degradation are therefore essential. Other applications where heat resistant tie-layer adhesives are necessary are for heat shrinkable multi-layer films used for cook-in applications. Still another application requiring good high temperature performance is for tubing for plumbing applications where the tubing can also undergo repeated heating and cooling cycles and the associated stresses resulting therefrom.
Composite polyolefin/metal pipe and tubing is extensively used for plumbing and heating systems. Polyethylene (PE) and crosslinked PE (PEX) and aluminum are most commonly used for these applications. These constructions typically would have the structure PEX/tie-layer/metal/tie-layer/PEX, PE/tie-layer/metal/tie-layer/PE or PE/tie-layer/metal/tie-layer/PEX. Even though the metal provides most of the mechanical strength in these constructions, mechanical integrity and pressure ratings are dependent on the quality of the bond between the metal and PE or PEX. It would be highly desirable if adhesive compositions having improved metal adhesion at elevated temperatures were available for these applications.
Ethylene copolymers containing vinyl silane comonomers, either grafted or copolymerized, are known. The presence of silane functionality in the polymers is a recognized means of obtaining crosslinkable compositions which are useful for a variety of applications. Silane crosslinkable polyolefins are widely used as insulation materials for low voltage power cables. Vinyltrimethoxysilane (VTMOS) and vinyltriethoxysilane (VTEOS) are the most commonly used silanes for copolymerization with ethylene or for grafting onto polyethylene backbones.
Copolymers of ethylene and VTMOS and terpolymers of ethylene, butyl acrylate and VTMOS have also been shown to exhibit good adhesion to aluminum. L. Ulren et al (Journal of Applied Polymer Science, Vol. 37, 1269-1285 (1989)) have shown that the introduction of silane groups in the polymer chain of polyethylene improves adhesion.
In copending application Ser. No. 10/299,282, filed Nov. 19, 2002 now U.S. Pat. No. 6,835,777, the use of organic bases, carboxylic acids and organometallic compounds are added to polyolefin-based adhesive blends to improve adhesion.